I had a close friend who was a sniper in Vietnam. He was sick for three days after confirming his first kill. It ruined him for life.
He’s now dead from cirrhosis of the liver, the the side effect of the self medication of choice by many who carry such burdens in their soul.
Men like your friend literally sacrifice part of their being for the benefit of the brotherhood and country. True heroes.
Other men, heroes too, are affected by it in different ways. For some, like this Army Ranger (and a better example would be Navy SEAL Chris Kyle), they internalize the killing as part and parcel of the mission, and they fully rationalize it as a case of the means justifying the end. A very interesting book that just came out on Amazon called 'Modern American Snipers' that looks at a variety of snipers from the US special operations forces, ranging from Delta to DEVGRU (SEAL Team 6) to Army Special Forces (Green Berets), 'regular' SEAL teams, Army Rangers, MARSOC, etc. Several of those snipers, such as Chief Petty Officer Chris Kyle (hereafter referred to as CPO Chris Kyle), in essence end up thriving in the mission. There's one famous case where CPO Chris Kyle takes a knife a guts a dog that had been adopted by his SEAL Team 3 sniper element because the dog would bark the whole night through and prevent CPO Chris Kyle from sleeping. His rationale? He needed proper sleep to be able to do missions the next day, and considering his missions meant saving the lives of his fellow soldiers that more than justified the life of the dog. That logic makes 100% sense when looked at from that perspective, and shows that people like CPO Chris Kyle saw whatever it was that they did as simply a mission to save lives by killing as many of the enemy as humanly possible (and while Nicholas Irvin killed a lot of men who would have been shooting at American soldiers, CPO Chris Kyle had confirmed kill numbers that were multiples of that).
In all cases, men like your friend and these other snipers are true heroes. Men like your friend, Army Ranger Nicholas Irvin, and Navy SEAL Chris Kyle have undoubtedly saved MANY lives, often at great personal cost.
As for the dog incident, the book states it best:
"A lot of people were like, 'Holy S#!t!' We need to give this guy a psych evaluation,'" Davis said. "Chris admitted that people freaked out about it. But I would say, no, that is simply someone who is 100 percent mission focused. His job is to go out and save the lives of other Americans and he knew that this dog could stop him from doing that. He was able to put his feelings and conscience aside to do what needed to be done. There are not a lot of people that can do that anymore."